This invention, in general, relates to battery charge sensing circuits, and more particularly to, monitoring circuits for a battery charger.
Battery chargers for nickel cadmium batteries have been in use for many years and are well known by one skilled in the art. Battery chargers of this type were extremely simple in design and manufacture. Circuitry included of a full wave bridge rectifier that coupled directly to the battery. The voltage of a charging battery would eventually increase until its voltage was greater than or equal to the full wave bridge at which point charging would cease.
There are more sophisticated chargers that rapidly charge both Nickel Cadmium and Nickel Metal Hydride batteries. A typical feature of these types of chargers is that they monitor a change in battery voltage or a change in battery temperature while charging to determine when charging should be terminated. The battery is charged with a constant current., thus the battery voltage or temperature is monitored as a function of time to determine rates of change.
Nickel Cadmium and Nickel Metal Hydride batteries have a very low internal impedance, for example, 50 milliohms. High charging currents do not produce significant changes in measured battery voltage when either charging or not charging due to the low internal impedance. Typically, the battery voltage is monitored while continuously charging the battery. The charging voltage remains flat for a portion of a charging cycle. The battery voltage then rises, plateaus, and finally decreases. It is this decrease in charging voltage as a function of time that signals termination in the charging cycle.
A second method commonly used to determine a charge status of a battery is to monitor the temperature of a battery as a function of time. Nickel Cadmium and Nickel Metal Hydride batteries have a characteristic that when they reach the end of the charge period the temperature of the battery increases greatly. This sudden increase in temperature is monitored and used to end the battery charging cycle.
In the quest for reducing battery cost, increasing reliability, and providing an environmentally friendly battery, rechargeable alkaline batteries have been developed. Rechargeable alkaline batteries have significantly different characteristics than rechargeable Nickel Cadmium or Nickel Metal Hydride batteries such that standard battery chargers cannot be used. One such characteristic is a high internal resistance which may range from 1 to 5 ohms. A high resistance would not yield an accurate measurement of the battery voltage during high current charging. Rechargeable alkaline batteries are charged with a pulsed current. Battery voltage is measured between current pulses (no battery charging) for accurate measurement. Traditional methods for determining when a battery is fully charged are not applicable to a pulsed charged system.
It would be of great benefit if a battery charger status monitor circuit could be provided which accurately determines when a battery is charged in a pulsed battery charger system.